A PLL is a device providing a signal synchronized with respect to a reference signal, that is, having the same phase and frequency as the reference signal. In the absence of a reference signal, the PLL provides a frequency approximately equal to the rated frequency of the reference signal. The function of the PLL is to follow the reference signal and to provide a signal similar to the reference signal in case of an accidental absence of the latter.
FIG. 1A shows a conventional simple digital PLL, intended to synchronize the pulses of an internal signal Fint with respect to the pulses of a reference signal SYNC. This PLL comprises a digital phase comparator (PHASE-COMP) 10 receiving signals Fint and SYNC and providing at its output a signal SGN. Signal SGN assumes a first logic state (1) if the phase of signal Fint is in advance to the phase of signal SYNC, i.e. if a pulse of signal Fint occurs prior to a pulse of signal SYNC, and assumes a second state (0) if signal Fint has a delayed phase. Signal Fint is obtained by dividing a clock signal CK through a divider (DIV N.sub.H -N.sub.L) 12 controlled by the signal SGN of the phase comparator. Divider 12 divides signal CK by a fixed high number N.sub.H when signal SGN is "1" and by a fixed low number N.sub.L when signal SGN is "0".
Number N.sub.H is such that the signal Fint obtained by dividing the clock signal CK by this number N.sub.H has a period higher than the rated period of the reference signal SYNC. Number N.sub.L is such that signal Fint has a period lower than the rated period of the reference signal SYNC.
FIG. 1B shows signals SYNC, Fint and SGN in an exemplary application of the PLL of Figure IA to TV signals, where signal Fint is used for generating the line scan. Signal SYNC is then generated by a receiving circuit or a magnetic tape recorder and its rated period is 64 microseconds.
Period T of signal SYNC, especially in the case of a video tape recorder where it is determined by the running speed of the tape, can vary within a substantially high range. Number N.sub.H is selected so that the corresponding signal Fint has a long period T.sub.H higher than the maximum period of signal SYNC, for example 65 microseconds. Number N.sub.L is selected so that the corresponding signal Fint has a short period T.sub.L lower than the minimum period, for example 63 microseconds.
FIG. 1B shows the signals obtained in steady state assuming that signal SYNC is at its rated period (64 microseconds).
At a time t0, a pulse from signal Fint is detected prior to a synchronization pulse from signal SYNC, which indicates that signal SYNC has a phase lag with respect to signal Fint. The signal SGN of the phase comparator goes to "1" and selects the division of the clock signal CK by the high number N.sub.H. The next period of signal Fint is therefore fixed to 65 microseconds.
At a time t1, the next synchronization pulse of signal SYNC is detected before a pulse from signal Fint. This indicates that the signal Fint has a phase lag with respect to signal SYNC. Signal SGN goes to "0" and selects the division by the low number N.sub.L and therefore a 63-microsecond period of signal Fint.
At a time t2, a pulse from signal Fint is detected prior to the next synchronization pulse from signal SYNC, indicating a phase lag. Signal SGN switches to state "1" and the process is resumed in the same way as at time t0.
In the above case where the period T of the reference signal SYNC is constant and equal to the average value of the long and short periods of signal Fint, the period of signal Fint oscillates at each period between 63 and 65 microseconds.
If the period T is slightly higher than 64 microseconds, the period of signal Fint oscillates in the same way with, however, from time to time, a few successive periods of 65 microseconds. If the period T is slightly lower than 65 microseconds, signal Fint will have a period of 65 microseconds with, from time to time, a period of 63 microseconds. In fact, the long or short periods of signal Fint successively occur so that the average frequency calculated over a large number of periods of signal Fint is equal to that of signal SYNC. It can be demonstrated that the duty cycle of signal SGN is expressed by (T-T.sub.L)/(T.sub.H -T.sub.L).
If period T is higher than 65 microseconds, the period of signal Fint is blocked at 65 microseconds and is therefore false.
Complementary cases occur when period T ranges from 63 to 64 microseconds.
Thus, this PLL is capable of following a reference signal SYNC only if its period T ranges within determined limits (here, 63 and 65 microseconds). It is therefore advantageous to separate as much as possible the extreme limits of signal Fint in order to increase the tracking range.
However, by increasing this range, the resolution of the PLL is reduced. In fact, assuming that signal SYNC is the line scan synchronization signal of a TV set, the period jumps between the extreme periods of signal Fint, from which scanning is generated, appear in the form of lines on the screen alternatively beginning forward or backward one with respect to the other. If the extreme periods of signal Fint are too high, these line differences become visible and unpleasant (a vertical line will be indented).